1. Field of the Invention
The present invention relates to a method for forming a polished surface of excellent smoothness on the surface of a vapor-phase synthesized thin diamond film (hereinafter referred to as a xe2x80x9cthin diamond filmxe2x80x9d).
2. Description of the Background
It is well-known that an X-ray mask deposited on a semiconductor surface for the preparation of an integrated circuit on the semiconductor can be prepared by:
(a) forming a thin diamond film having an excellent X-ray transmissivity of a thickness of from 1 to 3 xcexcm by the known vapor-phase synthesis technique on the upper surface of an Si wafer (substrate) having a thickness of about 380 xcexcm;
(b) polishing the surface of the thus formed thin diamond film to a prescribed surface roughness; then
(c) forming a membrane composed of the thin diamond film by melting and removing the center portion of the Si wafer from the backside thereof by the use of an etching solution such as a mixture of hydrofluoric acid and nitric acid, thereby forming an Si frame;
(d) sequentially forming an undercoat film, comprising indium-tin oxide, for example, having a high permeability to visible light while preventing the build-up of electric charge caused by charged particles, an X-ray absorber of Wxe2x80x94Ti alloy (containing from 1 to 2% Ti), a metallic Cr film serving as an etching mask, and a resist film on the membrane applied by sputtering or spin coating;
(e) forming a pattern of an integrated semiconductor circuit by scanning the resist film with an electron beam;
(f) etching the metallic Cr film by exposure to a mixed gas of chlorine and oxygen, using the thus formed pattern as an etching mask; then
(g) in a state in which the Si frame is cooled to about xe2x88x9250xc2x0 C., low-temperature etching the X-ray absorber from the bottom thereof, thereby forming an integrated circuit pattern on the semiconductor; and
(h) finally removing the metallic Cr film.
For the purpose of polishing the surface of the thin diamond film in step (b) of the X-ray mask, the method which is generally used comprises the steps of applying a polishing liquid containing powdered diamond particles having an average particle size of about 3 xcexcm dispersed in a solvent comprising a mineral oil such as a light oil or a white kerosene at a ratio within the range of 0.1 to 3 wt. % to a thin diamond film;
applying pressure to the surface of the thin diamond film to bring the same into contact with the surface of a soft stool made of copper or tin;
spraying the polishing liquid onto an exposed portion of the soft stool; and
polishing the stool and/or the thin diamond film while maintaining the powdered diamond particles in a state in which the particles stick to the surface of the stool which contacts the thin diamond film, the polishing occurring by a repeated mutual plane displacement action such as a mutual horizontal rotation.
Recent advances in the field of semiconductor devices are leading increasingly to higher degrees of integration of the devices, and along with this tendency, X-rays which pass through a thin diamond film which composes the membrane of the X-ray mask are required to be free from a shift which would result in a positional shift of an integrated circuit which receives the radiation of this shift. The X-ray shift is largely affected by the smoothness of the polished surface of the thin diamond film. A lower smoothness leads to a larger X-ray shift. Consequently, the thin diamond films must have increasing greater smoothness. With the conventional polishing method as described above, however, the achievable smoothness of a polished surface of the thin diamond film is an Rms (mean square surface roughness) of about 30 nm at most. Polished surfaces having a surface roughness of this order cannot sufficiently cope with the more extensive integration of semiconductor integrated circuits.
In view of these considerations, studies have now been conducted with a view to achieving a further improvement in smoothness of the polished surfaces of thin diamond films, and the following findings have been made. It is possible to achieve further smoothing of the polished surfaces of thin diamond films, as typically represented by an Rms surface roughness within a range of from 0.5 to 10 nm, in contrast to an Rms of only 30 nm in the conventional polishing method, by using, as a polishing liquid, an aqueous solution in which silicon-dioxide (hereinafter referred to as xe2x80x9cSiO2xe2x80x9d) powder particles having an average particle size of from 5 to 1,000 nm are dispersed and distributed in the aqueous solution in an amount of from 5 to 40 wt. %, the aqueous solution having a coefficient of viscosity of from 1 to 200 cP and a pH of from 8 to 12.5; pressing the surface of the thin diamond film and bringing the same into contact with the flat surface of a stool made of an artificial leather such as soft foamed polyurethane, a fabric such as a non-woven felt fabric, artificial or natural soft organic material such as rubber, a synthetic resin or wood, or preferably, soft foamed polyurethane or non-woven felt fabric; and polishing the surface of the thin diamond film by repetitive mutual plane displacement of the stool and/or the thin diamond film.
Accordingly, one object of the present invention is to provide a method of polishing the surface of a vapor-phase deposited, synthetic thin diamond film to a greater degree of surface smoothness.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a method of polishing the surface of a vapor-phase deposited, synthetic thin diamond film, comprising the steps of:
preparing a polishing liquid of silicon dioxide powder particles having an average particle size within the range of from 5 to 1,000 nm dispersed and distributed in an aqueous solution in an amount of from 5 to 40 wt. %, said dispersion having a coefficient of viscosity of from 1 to 200 cP and a pH of from 8 to 12.5,
applying the polishing liquid to a surface of a vapor-phase synthetic thin diamond film and bringing a flat surface of a stool composed of a soft artificial or natural organic material into contact with the surface; and
repetitively applying a mutual planar movement under pressure between the surface of the stool and the surface of the thin diamond film.
The reasons for limiting the average particle size of the SiO2 particles and the amount of the particles in the dispersion of the polishing liquid, and the coefficient of viscosity and pH of the polishing liquid in the method of the present invention are as now described.
(a) Average Particle Size of SiO2 Powder
A finer particle size of SiO2 powder leads to the preparation of a polished surface of a higher smoothness. An average particle size of under 5 nm causes a sudden decrease in polishing efficiency, thereby requiring a longer period of time to prepare a prescribed polished surface. At an average particle size of over 1,000 nm, on the other hand, it becomes difficult to prepare a polished surface having a surface roughness better than Rms: 10 nm. The average particle size of SiO2 powder should, therefore, be within the range of from 5 to 1,000 nm, preferably from 10 to 100 nm.
(b) Dispersion Ratio of SiO2 Powder
At a SiO2 particle amount of less than 5 wt. %, the dispersion concentration of SiO2 in the aqueous dispersion is lower, thereby requiring a longer period of time to prepare the desired surface polish. This is not, therefore, practicable. At an amount of over 40 wt. %, on the other hand, aggregation occurs between powder particles, and thie presence of aggregated SiO2 powder particles impairs smoothness. The ratio should, therefore, be within a range of from 5 to 40 wt. %, preferably from 10 to 20 wt. %.
As to the SiO2 powder, preferred are fumed silica and colloidal silica usually manufactured by the vapor-phase synthesis method or the liquid-phase synthesis method. In this case, the surfaces of SiO2 powder particles may be fully oxide surfaces on the atomic level, or the surfaces may be terminated with hydrogen or fluorine may be substituted for oxygen on the surface.
(c) Viscosity of Polishing Liquid
The viscosity of the polishing liquid has an effect on the polishing efficiency. That is, a polishing dispersion of too low a viscosity can be adjusted by the addition of ethylene glycol and a dispersion of too high a viscosity by the addition of ethylene glycol and glycerin for example. A coefficient of viscosity of under 1 cP results in a sudden decrease in polishing efficiency, and a coefficient of viscosity of over 200 cP leads to a long residence time of SiO2 powder on the polished surface, thus acting to inhibit smoothing of the surface. The coefficient of viscosity should, therefore, be within a range of from 1 to 200 cP, preferably from 10 to 30 cP.
(d) pH of Polishing Liquid
While dispersibility of SiO2 powder particles in the polishing liquids varies with the pH value of the dispersion, a pH value of under 8 or of over 12.5 leads to a decrease in dispersibility of the SiO2 particles. If a polishing liquid having such a decreased dispersibility is employed, it is impossible to form a polished surface having excellent smoothness. In order to ensure uniform dispersion of SiO2 particles, it is necessary to adjust the pH value of the polishing liquid, usually with KOH, and further, with NH4OH , NaOH or other alkaline substance within the range of from 8 to 12.5, preferably from 9.5 to 11.
Having now generally described the invention, a further understanding can be obtained by reference to certain specific Examples which are provided herein for purpose of illustration only and are not intended to be limiting unless otherwise specified.
First, a thin diamond film having a thickness of 3.5 xcexcm was formed, by the microwave method, which is a known vapor-phase synthesizing method, on the upper surface of each of ten Si wafers (substrates) each having a radius of 75 mm and a thickness of 1 mm. These thin diamond films had a surface roughness of a Rms value within the range of from 78 to 85 nm.
KOH was mixed into an extra-pure water, and its pH was adjusted. SiO2 powder particles having the average particle sizes shown in Table 1 were dispersed and distributed at the various ratios shown also in Table 1. Viscosities too low were adjusted with ethylene glycol, and viscosities too high were adjusted with ethylene glycol and glycerin, to prepare polishing liquids having a necessary coefficient of viscosity and pH as shown in Table 1.
Then, a stool having a flat surface diameter of 300 mm and a thickness of 10 mm and having the surface lined with a polishing pad made of an artificial leather or soft foamed polyethylene was immersed horizontally in the above-mentioned polishing liquid and held in it. Each of the above-mentioned thin diamond films on Si wafers was placed, with the surface in contact with the polishing pad on the upper surface of the stool immersed in the polishing liquid, and a pressure was applied onto the Si wafer via a holder by means of a hydraulic cylinder. In this state, Examples 1 to 13 of the invention were conducted by polishing the surface of the thin diamond film by mutually rotating the stool and/or the Si wafer under the polishing conditions shown in Table 1. The surface roughness (Rms) of the polished surfaces were measured. The result of this measurement is also shown in Table 1.
For comparative purposes, diamond powder particles having the average particle sizes shown in Table 2 were dispersed and distributed in a white kerosene at ratios also shown in Table 2, thereby preparing polishing liquids.
Then, each surface of the thus prepared thin diamond films on Si wafers was placed in contact with the upper surface of a copper stool. A pressure was applied via a holder onto the Si wafer with a hydraulic cylinder, and the conventional methods 1 to 3 were conducted by polishing the thin diamond film surface by mutually rotating the stool and/or the Si wafer under the polishing conditions shown in Table 2 while spraying the polishing liquid onto the stool. The surface roughnesses (Rms) of the polished surfaces were measured. The results of these measurements are shown in Table 2.